In optical telecommunication transport networks different optical wavelength channels can be multiplexed into a single fiber. FIG. 1 shows a transmitting node (1) with a multiplexer (MUX) and transmitters (3) for inserting desired data and pilot signals into the fibers. The figure also shows a receiving node (2) with a demultiplexer (DEMUX) and receivers (4). Optical wavelength channels carrying data can be dropped or added in the nodes of the network, i.e. the nodes comprise transmitting and receiving elements. For monitoring which channels are present at a given place in the network, pilot tones specific to each wavelength channel can be superimposed (3) on the wavelength carriers. The carriers can be detected by tapping (5) a portion of the light, typically 10 percent, in a given place into a photodetector. To be more precise, the optical tap (5) diverts a small part of the optical power for extracting desired pilot tones (or a tone). The optical tap (or taps) can be located before (5) the demultiplexing (2) of the channels, after the demultiplexing, but before the receivers, or in the receivers.
FIG. 2 shows an example of an arrangement for detecting a pilot tone in the fiber (6). The pilot tone can be detected by filtering the signal obtained from a photodetector (13) through an optional bandpass filter (8) tuned to let through the frequency range of the pilot tones and to block most of the telecommunications data signals. It is also possible to tune the filter for letting only the desired pilot tone through. Usually, the signal from the photodetector is amplified (7) before the bandpass filter. If the pilot tone has a known modulation depth, the power of the optical carrier can be measured by measuring the amplitude of the pilot tone. Often, the measuring device (11) is a piece of digital equipment, such as a digital signal processor. Due to this the signal from the bandpass filter must be converted to a digital format. The converter (9) used gets a sampling rate from an adjustable oscillator (10). The management system (12) can use the measured amplitude of the pilot tone for any purposes required. Using pilot tone facilitates the detection of any purposes required. Using pilot tone facilitates the detection of the presence or absence of wavelength carriers, because neither optical filters nor the examining of the telecommunication data signal are required if pilot tones are used.
The pilot tone amplitude has to be low so as not to disturb the data signal. In addition, the data signal itself requires a broad frequency spectrum, starting from a few tens of kilohertz up to a few gigahertz. Furthermore, there can be tens or hundreds of wavelength channels of different power levels. Detecting weak pilot tones in an aggregate signal incorporating a multitude of other pilot tones, as well as noise due to the numerous data channels, is a challenge to the detection system. Since multiple fibers are connected to each node, the detection system should be as simple as possible.
A very sensitive method for detecting a pilot tone is to use a phase-locked loop in a receiver containing a local oscillator that is tuned to the pilot tone frequency. The signal from the photodetector multiplied by the local oscillator yields, after low-pass filtering, the amplitude of the pilot signal. By using the product signal as feedback into the local oscillator, the phase of the local oscillator can lock onto the pilot tone phase. In this solution, detecting tens of channels may be problematic because it takes time for the receiver to lock onto the phase of the received signal.
Another method is to sample and digitize the signal. The Fast Fourier Transform (FFT) technique is then applied on the data to extract the pilot tone amplitudes. Normally, calculating fast Fourier transform requires quite great processor power because a significant number of multiplications have to be carried out. For example, for a data set of 4096 samples, 49000 multiplications (with complex numbers) have to be carried out. If a powerful processor were not needed, cost savings would be obtained.
The objective of the invention is to avoid these drawbacks by minimizing the number of calculations while maintaining high sensitivity for detecting a pilot tone. This is achieved in a way described in the claims.